The invention relates generally to the recognition and solving of a previously unrecognized problem inherent in an optical imaging head that uses a diffractive modulator in a platesetter. More specifically, banding and other imaging artifacts are minimized.
With any image to be printed in the printing industry, a typical first step in the overall process is prepress operations, that is, to transform digital information representing the image of interest onto a light or thermal sensitive medium, e.g. a printing plate, which is then used to transfer the image any number of times on a printing press. As prepress technologies have evolved, the time required to create the image has been reduced while the quality of the image has improved. Also evolving is the media used for film, plates, proofing and final production. This evolution continues to drive the requirement for a faster, higher quality, imaging system capable of imaging on many different recording media.
One method for reducing imaging time is to increase the number of beams that write on the media at any one time. There exists in the art several methods for creating multiple writing beams from a single source. These previous methods include the use of a multi-channel acousto-optical modulator (AOM), various beam splitting technologies and multi-element modulators such as digital micromirror devices (DMD™ trademarked by Texas Instruments), and lanthanum modified lead zirconate titanate which can be fabricated as a transmissive ferroelectric ceramic modulator, also known as PLZTs.
One type of multi-element diffractive modulator, the grating light valve or GLV™, has been developed by Silicon Light Machines, Inc. of Sunnyvale, Calif. This GLV is an addressable diffraction grating, formed of moving parts on the surface of a silicon chip. Each GLV pixel consists of dual-supported parallel ribbons formed of silicon nitride and coated with a reflective aluminum top layer. Several publications are incorporated herein by reference in their entirety to provide supplemental background information on grating light valves which is not essential but is helpful in appreciating the applications of the present invention. They are: “Grating Light Valve™ Technology: Update and Novel Applications” by D. T. Amm et al., presented at Society for Information Display Symposium, May 19, 1998, Anaheim, Calif.; “Grating Light Valve™ Technology for Projection Displays” by R. W. Corrigan et al., presented at the International Display Workshop, Kobe Japan, Dec. 9, 1998, Paper Number LAD5-1; “Optical Performance of the Grating Light Valve Technology” by D. T. Amm et al., presented at Photonics West-Electronic Imaging, Jan. 27, 1999, San Jose, Calif.; and “Calibration of a Scanned Linear Grating Light Valve™ Projection System” by R. W. Corrigan et al., presented at Society for Information Display Symposium, May 18, 1999 in San Jose, Calif.
U.S. Pat. No. 6,229,650 issued May 8, 2001 to Reznichenko et al. discloses an optical imaging head for transferring an image onto a printing plate mounted on a drum surface of an internal or external drum platesetter. The optical imaging head as disclosed in the '650 patent includes: a line illumination module; a grating light valve; a first lens group; a second lens group; and a stop. The line illumination module generates a substantially uniform line of radiation and includes a bar of laser diodes, a fast axis collimating lens for evenly dispersing radiation in a fast axis direction, and a slow axis collimating lens for evenly dispersing the radiation in a slow axis direction. The grating light valve forms an object plane which receives the line of radiation from the line illumination module and generates diffractive orders of modulated radiation. The grating light valve includes (i) an addressable diffraction grating formed of moving parts on the surface of a silicon chip, and (ii) pixels of dual-supported parallel ribbons formed of silicon nitride and coated with a reflective aluminum top layer. The first lens group receives the modulated radiation and adjusts image magnification independent of image focus. The second lens group receives and passes the magnification-adjusted modulated radiation from the first lens group to the printing plate, and adjusts the image focus independent of image magnification. The stop, which is placed between the first and second lens groups, has a single aperture for (i) passing zero order diffractive magnification-adjusted modulated radiation from the first lens group to the printing plate, and (ii) blocking non-zero order diffractive magnification-adjusted modulated radiation from incidence with the printing plate.